The pulmonary edema is non-cardiogenic in origin . The diagnosis is mainly made on clinical grounds . The American-European Consensus Conference has set forth the following criteria to make a clinical diagnosis of ARDS (Conrad , 2005

4 . PaO2 /FIO2 ratio less than 200 (ARDS ) or less than 300 (acute lung injury [ALI] : ALI is a milder clinical expression of the injury of ARDS that may or may not progress to ARDS (Conrad , 2005

Etiology

There are many predisposing conditions leading to ARDS . These include

1 . Infections . Pneumonia of any etiology can predispose to the condition . Viral pneumonia is the commonest type of pneumonia leading to ARDS . Gram negative systemic sepsis also can predispose to ARDS (Conrad 2005

2 . Septic and traumatic shock can predispose to ARDS . Infact , any type of shock can lead to this condition

3 . Aspiration of either gastric contents or water during drowning can lead to ARDS

The hallmark pathology in this condition is diffuse alveolar damage . There is also damage to the lung capillary endothelium (Harman Walia , 2006 . This leads to loss of integrity of the alveolar-capillary barrier . There is transudation of protein-rich fluid across the barrier resulting in pulmonary edema . Intrapulmonary shunting occurs leading to hypoxemia (Conrad , 2005

The pathogenesis of ARDS can be described in 3 phases

1 . Exudative phase : This is the initial phase in which there is injury to the endothelium and epithelium leading to inflammation and fluid exudation

2 . Fibroproliferative phase : In this phase which follows the initial phase , there is influx and proliferation of fibroblasts and other cellular elements . The patient may recover or worsen in this stage . In sepsis , the main focus of injury is in the vascular endothelium . In aspiration of gastric contents , the focus of injury is in the alveolar epithelium (Harman Walia , 2006 . Neutrophils play an important role in the pathogenesis of ARDS (Harman Walia , 2006 . The neutrophils trigger release of cytokines , such as tumor necrosis factor (TNF leukotrienes and macrophage inhibitory factor . Platelet sequestration and activation is also noted (Harman Walia , 2006

3 . Fibrosis : This phase occurs in those who have survived ARDS . There is resolution of inflammation and development of pulmonary fibrosis (Conrad , 2005

Clinical features

The obvious clinical symptom in ARDS is dysnoea . The patients present with labored breathing , tachypnea , incresed work of breathing and hyperventilation . Due to hypoxemia there may be cyanosi...